As described in U.S. Pat. No. 4,378,812 a standard blood-collecting kit comprises a sampling tube or vial having an open end, a cap fitted over this end and having a pierceable membrane, and a needle holder that is in turn fitted to the cap. The needle assembly comprises a luer collar fitted with a double-ended cannula or needle whose front end is inserted through the patient's skin normally into a vein from which blood is to be drawn. The cap also has a luer fitting that fits complementary with the collar of the needle with the rear end of the needle poking through the membrane, and the opposite side of the cap is normally threaded to fit on the collection tube. When fully assembled with the front needle end inserted into a blood vessel, a piston in the collection tube is retracted to draw blood through the cannula into the tube. Once the tube is full, it can be unscrewed and another tube similarly filled, if desired.
When the holder is separated from the cap some blood often drips from the rear needle end onto the membrane. Since this membrane is invariably mounted at the extreme outer end or mouth of a tubular extension of the cap, this creates a substantial risk of contamination and provides a possibility of a medical worker coming into direct contact with the warm fresh blood thus dripped onto the exposed face of the membrane. In this day and age it is obvious that such a potentially dangerous situation must be avoided.
In order to eliminate this problem it has been suggested to provide the cap with a pierceable membrane or plug which is mounted at the outer end of the cap but that has an outwardly concavely recessed floor so that any drip does not simply lie on a flat end surface, but is slightly recessed. This type of plug reduces the already small diameter of the cap extension or the wall thickness of the extension is increased by the wall of the plug. This recess creates a reduction in the diameter of the cavity into which the needle is poked, which creates extra friction with the pierceable sterile sheath that normally surrounds the rear end of the needle and that is pushed back like a harmonica as the needle pierces it and the membrane of the cap. Furthermore when the needle assembly is pulled off the cap, any blood on the needle tip is transferred to the end of the bunched-up sheath and spread on the end of the cap. Furthermore if the blood tube is knocked over, any blood on the outside of the membrane is free to run out of the tube, once again creating a possibility of infection.
In order to avoid some of these difficulties it has been suggested to clamp the membrane rather forcibly in the cap. This reduces leakage and prevents it from being pulled out when the needle is extracted. On the other hand the extra compression of the elastomeric disk makes the membrane somewhat harder, requiring more force to be exerted to poke the needle through it, and causing the needle to exert more traction on it when being pulled out. Of course pulling the membrane out of the cap creates a serious problem of contamination and spoils the sample.